y = x = (S'xs' + Dxd)/(S' + D), which is the mean molar composition of the overhead product and sidestream. Since xs> < xd, and Ls < Ln, this additional operating line cuts the y = x line at a lower value than the upper operating line though it has a smaller slope, as shown in Figure 11.23. The two lines intersect at x = xs/. Plates are stepped off as before between the appropriate operating line and the equilibrium curve. It may be seen that the removal of a sidestream increases the number of plates required, due to the decrease in liquid rate below the sidestream.

The effect of any additional sidestream or feed is to introduce an additional operating line for each stream. In all other respects the method of calculation is identical with that used for the straight separation of a binary mixture.

The Ponchon-Savarit method, using an enthalpy-composition diagram, may also be used to handle sidestreams and multiple feeds, though only for binary systems. This is dealt with in Section 11.5.

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